With the development of smart devices, many display devices, such as portable electronic devices and automotive equipment, are being used in our daily lives. In order to allow a display device to provide clear and non-glare images under different conditions, a conventional display device adjusts image brightness by adjusting an intensity of light emitted by a light source, and the light intensity of the light source is adjusted according to an intensity of ambient light. In a display device using light emitting diodes (LEDs) as the light source, the intensity of light is adjusted by a drive current in the LEDs.